1. Field of the Invention
This invention is generally related to fluid flow controllers and more specifically to mass flow controllers for gas flowing at very low rates and at low pressures.
2. State of the Prior Art
There are many flow controllers, including mass flow controllers, of myriad designs and operating principles for measuring flowing gases. However, current flow control technologies for very low gas flow rates leave much to be desired in accuracy, reliability, and durability, even though the need for such very low rate flow controllers for gases is increasing. In the semiconductor industry, for example, very precise flows of feed gases are flowed into reaction furnaces that usually operate in a vacuum, where thin films of materials are deposited on substrates to form semiconductor devices. The most common mass flow controller technology that is used currently in the semiconductor industry for controlling feed gases is a variable orifice in combination with a flow restrictor and a bypass around the flow restriction, a heater in the bypass, and a thermocouple in the bypass downstream from the heater. The heater imparts thermal energy to the gas that flows through the bypass, and the gas carries the thermal energy to the thermocouple, which heats the thermocouple and causes it to produce voltages that are indicative of the temperature of the thermocouple. The more the variable orifice is opened, the more gas will flow through the bypass, the more heat will be carried by the gas from the heater to the thermocouple, the higher the resulting thermocouple temperature, and the higher the voltage will be across the thermocouple. The thermocouple voltage is measured and processed in instrumentation to indicate the flow rate of the gas and to adjust and maintain the variable orifice at openings required for desired gas flow rates. These current flow controller systems suffer from reliability problems, slow response times, and limited dynamic ranges. Reliability problems are due in large part to hard failures, such as closing, and to soft failures, such as excessive drift, which requires frequent recalibrations in the instrumentation. Such failures cause substantial down time and resulting decrease in yields of semiconductor devices from the reaction furnaces. The control problems result from slow response of the thermocouples to changes in flow rates, usually about one second, thus slow feedback of opening or closing signals to the variable orifices and resulting overshoots and undershoots of orifices required for particular gas flow rates. Operating pressures in ranges of 20-25 p.s.i. are required to maintain repeatability and linearity of flow rate measurements and control, and even then, linearity and repeatability of .+-.1% and usually more like .+-.6% is about all that can be expected with this technology. Dynamic range of such controllers, which is also known as turndown ratio (the ratio of maximum measurable flow rate or maximum set point of the valve to its minimum measurable flow rate or minimum set point), is limited to about 100:1.
More accurate and reliable mass flow controllers for feed gases in the semiconductor industry could not only increase quality controls and resulting semiconductor device quality, but could also decrease down time for recalibrating, cleaning, increase gas utilization, and increase yields. Larger dynamic ranges would accommodate more options, uses, and versatility of facilities and gas flows for different depositions and device compositions. More accurate and reproducible gas flow controls, better reliability, and larger dynamic ranges are also needed for many other applications as well.